1. Field of the Invention
The present invention relates to a magnetic device. The magnetic device of the present invention is useful in variety of applications such as magnetic memories, magnetic sensors, spin calculation devices, and so forth. In particular, the magnetic device of the present invention is useful as a part of solid magnetic memory devices.
2. Related Background Art
Conventionally, DRAMs, SRAMs, flash memories, EEPROMs, FeRAMs (or FRAMs) are used as the solid memory device. In recent years, magnetic solid memories, especially memories utilizing a TMR effect or a GMR effect, are attracting attention and are being investigated. Therefore, the solid magnetic memory which is relevant to the present invention is explained below.
(Giant Magnetoresistance (GMR))
Firstly the giant magnetoresistance relevant to the present invention is briefly explained. In 1986-1988, Fert et al., and Grunberg et al. found a magnetoresistance much higher than the known AMR in a ferromagnetic (Fe)/nonmagnetic (Cr) artificial lattice, and named this “giant magnetoresistance”. The GMR exhibits characteristically a negative resistivity change rate of as much as several-ten %.
The mechanism of the GMR is understood qualitatively as below. In an artificial lattice, in the absence of a magnetic field, the magnetic layers are arranged antiferromagnetically (interlayer antiferromagnetism). On application of a magnetic field, the magnetization of the layers are parallelized. In this process, the electric resistance is decreased by a spin-depending mechanism in which conduction electrons are remarkably scattered in an antiparallel magnetization state and weakly scattered in a parallel magnetization state.
The interlayer antiparallel magnetization is theoretically studied by employing an RKKY type long-distance exchange interaction or a quantum well model. The spin-dependent scattering between the layers is discussed on the basis of a two-fluid model of the conduction electrons.
The GMR effect can be utilized for a device such as a memory device. In such a device, the magnetization direction of a part of the magnetic layers is fixed, and the magnetization direction of other layers is made changeable for memorization. The device of such a constitution is called a spin-valve type device. The layer fixed in a magnetization direction (the high coercivity layer) is called a hard layer; the layer changeable in the magnetization direction (the low coercivity layer) is called a free layer. Conversely, information is recorded in the hard layers, and the magnetization state (memory state) is read by the resistivity change of the free layer on reversal of the magnetic field.
As the types of the GMR, there are known CIP types, CPP types, and hybrid types thereof such as CAP types, and granular alloy types. Generally, CIP structure is widely studied owing to the ease of the production. However, the CIP type device, in which the current flows in parallel to the lamination interface, is capable of changing its resistivity in the range of about 40 to 50% owing to the conduction electrons not contributing the interface spin scattering.
On the other hand, the CCP type device, in which the current flows perpendicularly to the lamination interface, is capable of changing more remarkably the resistivity, sometimes exceeding 100%, since all electrons are exposed spin scattering corresponding to the spin state of the electrons and the Fermi velocity is increased by the energy gap resulting from the lamination structure. Therefore, the CCP type device has better characteristics basically. However, the CCP type device, in which the current is allowed to flow perpendicularly to the film face, can have an extremely low resistance, which requires extremely small sectional area.
In the case where cell itself is thin like the TMR cell described later, the writing can be conducted effectively by two upper and lower wires. However, in a CPP type GMR cell, particularly a cell in which lamination direction is perpendicular to the substrate, the upper and lower wiring causes difference in the magnetic field intensity between the outer lamination layer portions and the inner layer portion, which may retard effective writing. In other words, the magnetic field is weak in the inner portion of the lamination cell, so that the writing may be hindered in the inner portion of the lamination layers, or an intense writing current may cause writing in a non-selected cell portion, disadvantageously.
In one method to solve this problem, an electric current is allowed to flow through the CPP cell to form a rotational magnetization by the self-magnetic field generated by the electric current. This method is applicable to somewhat larger cells, and may cause disturbance of the magnetization by the reading current, disadvantageously.
(Tunnel magnetic memory (TMR))
Memory cells utilizing a tunnel junction are generally of a spin-valve type as disclosed in Japanese Patent Application Laid-Open No. 10-190090. Such a memory device is explained by reference to FIG. 6.
In FIG. 6, memory cell 61 has a built-up layer structure constituted of a pin layer, an insulation layer, a memory layer, and so forth. The numeral 62 indicates a B-wire; 63, a G-wire; 64, a W-wire; 65, a MOSFET; and 66, a pass transistor. The magnetization in the magnetic layer is usually directed to one of the longitudinal axes.
The tunneling current is more intense and the cell resistivity is lower when two magnetic layers holding an insulation layer therebetween are magnetized in the same direction, whereas the tunneling current is weaker and the cell resistivity is higher when two magnetic layers holding an insulation layer are magnetized in opposite directions.
Usually, the magnetization direction is fixed in one of the two magnetic layers (a hard layer), and is changeable in the other magnetic layer (a free layer). The magnetization direction of the free layer is controlled and maintained by the magnetic field generated by the current flowing through the B-wire and/or the W-wire. That is, the writing is conducted only into the selected cells by a vector sum of the magnetic field generated by the B-wire and the magnetic field generated by the W-wire.
Read-out is conducted by the B-wire and the G-wire. The selection of the cell is made by a MOSFET connected to the G-wire.
The resistivity change rate of the TMR type material can be increased unlimitedly in calculation. However, the actually realizable level is about 40 to 60%. In production thereof, the most important problem in production and the characteristics of the device is a production method of the insulation layer and the bias dependency of the resistivity change rate. The insulation layer should be formed uniformly in a thickness of 1 nm, which is not easy in the practical device production. Furthermore, the bias dependency may become a problem by which the resistivity change rate drops remarkably at a higher voltage. These problems do not arise in the aforementioned GMR device.
The CPP type GMR structure employed in the present invention requires fine pores (minute through-holes) of higher aspect ratio. This structure can be obtained by a membrane filter prepared by track-etching, or by anodic oxidized alumina. The most suitable anodic oxidized film is explained below in detail.
(Anodic Oxidized Alumina)
An anodic oxidized alumina layer, a porous type anodic oxidation film, can be formed by anodic oxidation of an A1 plate in an acidic electrolytic solution such as sulfuric acid, oxalic acid, and phosphoric acid. (See, for example, R. C. Furneaux, W. R. Rigby, and A. P. Davidson: NATURE, Vol. 337, p. 147 (1989), etc.) This porous film is characterized by a specific geometrical structure in which extremely fine cylindrical fine pores (nano-holes) 11 having a diameter of several nm to several-hundred nm are arranged in parallel at intervals of several-ten nm to several-hundred nm. The cylindrical fine pores has a high aspect ratio and sufficient uniformity in the sectional diameter.
The structure of the porous film can be controlled to some extent by the anodic oxidation conditions. It is known, for example, that the pore spacing can be changed to some extent by anodic oxidation voltage; the pore depth by anodic oxidation time; and the pore diameter by a pore-widening treatment. The pore-widening treatment is an alumina etching treatment, usually a wet etching with phosphoric acid.
For improvement of the fine pores of the porous film in the verticality, linearity, and independency, a two-step anodic oxidation is disclosed (Japanese Journal of Applied Physics, Vol. 35, Part 2, No. 1B, pp. L126-L129, 15 Jan., 1996). In this method, a porous coating film formed by anodic oxidation is once removed, and then anodic oxidation is conducted again to obtain a porous film having pores excellent in the verticality, linearity, and independency. This method utilizes the surface indents on the Al plate left after removal of the anodic oxidation film formed by the first anodic oxidation, in the second anodic oxidation, as the pore-formation initiation points in the second anodic oxidation.
Further, for improvement of control of the shape, spacing intervals and arrangement pattern of the fine pores in the porous film, a method is disclosed (Nakao: U.S. Pat. No. 6,139,713; and Masuda: Kotai Buturi (Solid Physics) 31, 493 (1996), in which pore initiation points are formed by using a stamper. In this method, a plate having plural projections on the surface is pressed against the surface of the Al plate to form dents thereon as the pore-formation initiation points, and the Al plate is anodized to form a porous film having pores with the shape, spacings, and pattern controlled sufficiently.
Instead of the honeycomb arrangement, the fine pores can be arranged concentrically (Ohkubo: Japanese Patent Application Laid-Open No. 11-224422). Besides the stamping process, the initiation points can be prepared by an FIB process (Focused ion beam process), and a photolithography.
Japanese Patent Application Laid-Open No. 10-283618 discloses a process of embedding a built-up magnetic layer having GMR properties into the aforementioned anodic oxidation alumina nano-holes. This disclosure describes the wiring on the pore surface for magnetic writing into the built-up magnetic material. However, the writing into fine pores of high aspect ratio has not been solved yet.
The aforementioned usual TMR type memory elements require a large amount of electric current for the writing, and the devices consume a large amount of power. Further, nonuniformity of the memory cells or nonuniformity of the writing wiring can cause erroneous writing into a non-selected cells or writing failure in the selected cells.
In the cells having a larger aspect ratio in the thickness direction, writing is sometimes not effectively conducted by the crossing upper and lower wiring.